1. Field of the Invention
The present invention relates to an optical receiving device, and in particular to an optical receiving device for providing high operating bandwidth.
2. Description of the Related Art
In a conventional optical receiving device, a parasitic capacitor of a photodiode affects operating bandwidth and sensitivity of the device. Generally, when a value of the parasitic capacitor is large, it is difficult for the optical receiving device to operate with both high operating bandwidth and high sensitivity, thus increasing the difficulty in designing the optical receiving device.
FIG. 1 is a circuit diagram of a conventional optical receiving device. A cathode of a photodiode D1 is coupled to a voltage source VR1, and an anode thereof is coupled to an input terminal of a transimpedance amplifier T1. The transimpedance amplifier converts an optical current signal detected by the photodiode D1 to a voltage signal, and outputs the voltage signal Vout1 to back-end devices for data decision. A value of a capacitor Cd1 is a parasitic capacitance of the photodiode D1, a value of a resistor Rin1 is an input impedance of the transimpedance amplifier T1, and a value of a capacitor Cin1 is an input capacitance of the transimpedance amplifier T1. The resistor Rin1, the capacitor Cin1, and the capacitor Cd1 determine the operating bandwidth of the optical receiving device. The operating bandwidth is represented by the following formula:   B1  =      1          2      ⁢              π        ·                  rin1          ⁡                      (                          cin1              +              cd1                        )                              
wherein B1 represents the operating bandwidth, rin1 the value of the resistor Rin1, cin1 the value of the capacitor Cin1, and cd1 the value of the capacitor Cd1.
If the resistance rin1 is a constant, when the capacitance cd1 increases, the operating bandwidth B1 decreases. If the operating bandwidth B1 is kept as a constant, higher capacitance cd1 requires lower resistance rin1. Therefore, as the capacitance cd1 increase, so do difficulties in design of transimpedance amplifier.
As mentioned above, because the cathode of the photodiode D1 is coupled to the voltage source VR1, capacitor Cd1 of the photodiode D1 directly affects the bandwidth of the optical receiving device. However, if the voltage signal of the cathode is kept in phase with the voltage of the anode, the influence of the capacitor Cd1 on bandwidth is substantially decreased.